This invention relates to the termination of cables, especially multiconductor cables.
The protection of electric wiring against the ingress of electrical interference is of special importance when high reliability electric systems of a complex nature have to function properly in an environment of high level electrical interference. The signal circuits in such systems are operated at low levels and must be adequately screened to prevent malfunctions caused by such interference.
In practical terms, the exclusion of interference from sensitive circuits entails surrounding the vulnerable areas with a conductive shield, usually connected to earth. Referring especially to cable harnessing, this means that there has to be employed some kind of braiding or flexible electrically conductive tubing with no openings in it and with a low electrical resistance in itself and in any connections which it contains. In practice, this may entail one or more braided screens and, taking a typical example, a multicore cable used in such a harness would comprise signal lines in the form of twisted pairs to reduce interference sensitivity by common mode rejection, each twisted pair being individually screened. The bundle of wires is then enclosed in an outer overall screen comprising one or more layers of braided wire protected by a flexible plastics outer jacket. The braids are conventionally made from copper, optionally plated with nickel, silver or tin.
Obviously, the termination of multicore cables screened in this way presents certain difficulties. For example, when the cables have to pass into adaptors, plugs and other connecting devices, not only do the individual conductors have to be properly connected to the respective device but also the individual screens and the outer screens, if present, must be properly grounded. The cables commonly comprise bundles and sub-bundles of conductors which may not be of uniform size and, until recently, the termination of such cables has necessitated the use of special tools and bulky termination devices. This has led to a high assembly cost.
An ideal termination device would meet the following requirements:
1. It would provide a means for individually terminating each of the inner screens in a way which avoided loops.
2. It would be easy to load without special tools.
3. It would be free of holes through which interference might leak.
4. It would be easily attached to a standard connector.
5. It would provide means for terminating the outer screen(s).
6. It would be relatively light and non-bulky.
7. It would be electrically sound but, at the same time re-openable for repair or modification purposes without damage to the screens so that the termination can be re-made without further cutting back of the cable.
In U.S. Pat. No. 3,465,092 there is described and claimed a termination assembly for bundles of electrical leads or screens which comprises a first cylindrical, externally threaded element having a plurality of spaced longitudinal slots and a second cylindrical driving ring in threaded engagement therewith. The driving ring rotatably carries a contact annulus which is provided with a plurality of contact sections which fit the slots of the first element. The individual leads or screens are positioned in the slots and the assembly is then screwed together tightly so that the leads or screens are firmly held in the slots by the contact sections of the annulus.
Whilst this assembly has many advantages in that it is relatively simple, non-bulky and does not require the use of special tools it does, however, have certain disadvantages. First of all, the final connection is formed within the second cylindrical element and so it is not possible to check visually that a proper connection has been made. Secondly, when a plurality of leads or screens are to be connected there are no means provided for holding one lead (or screen) in place whilst another is being loaded. Finally, there are no means provided for simply terminating an outer screen (or screens). The last disadvantage is especially important in high interference applications.